In WCDMA (Wideband Code-Division Multiple Access) HS-DSCH (High-Speed Downlink Shared CHannel) radio networks, the radio network controller (RNC) is responsible for ensuring that data is available in the radio base station (RBS) for transmission. To facilitate this, the RNC sends radio link control (RLC) data via a transport network (TN) to the RBS. Normally, the RNC controls the buffer fill level in the RBS by sending RLC protocol data units (PDUs) to the RBS via a TN frame protocol (FP). While the RBS sends capacity allocations (CA) as input to this traffic shaping, it is still the RNC that controls how much data and at which rate this shall be sent to the RBS.
It should be noted that since RLC is terminated in the RNC and in the user equipment (UE), any RLC data residing in an RBS queue is seen as delayed by the RLC entities involved in the data transmission. As a result of this, the RNC might poll timer expiry poll data that has been transmitted from the RNC but not yet acknowledged by the UE. Thus, for instance if the amount of data in the RBS priority queue (PQ) is large in relation to the aggregated rate over the air interface (Uu), then the RLC entity in the RNC might poll the RLC entity in the UE due to the buildup of a transmission delay even though no data has actually been lost. Consequently this can lead to unnecessary retransmissions of RLC PDUs or POLL_SUFIs (SUper FIelds) sent solely to poll the receiving RLC entity.
RLC retransmissions might also pose a potential challenge. If these are delayed by data already buffered in the PQ, this can lead to multiple requests for the same data even though the previously retransmitted data is already in transit but buffered in the RBS PQ. In this situation with delayed retransmissions, the UE may have time to send an additional RLC status report before the initial retransmission even has been transmitted to the UE over the air interface leading to a situation were multiple copies of the same data might be sent both over the TN and Uu interfaces. This results in an inefficient use of the TN and air interface resources, since the additional copies of retransmitted RLC PDUs do not contribute to the user experienced throughput since they will be discarded by the receiving RLC entity in the UE.